It is the base of a cross, a subcircular boulder measuring roughly 35 by 37 centimetres, with a rectangular socket cut into its upper surface. That socket, about 15 centimetres long, 7 centimetres wide, and 3 centimetres deep, is the essential detail: it is the void where a cross shaft once stood, held upright by the shaped stone beneath it. The cross itself is long gone, but the base preserves the geometry of its absence.

Inis Cealtra, also known as Holy Island, has been a place of Christian settlement since at least the seventh century, associated in tradition with Saint Caimin. The island carries the remains of several early churches, a round tower, and a collection of grave slabs, making it one of the more substantial concentrations of early medieval ecclesiastical material in the west of Ireland. Cross bases of this kind were functional objects as much as devotional ones, designed to keep a standing cross stable in exposed conditions. The rectangular socket form is characteristic of early Irish ecclesiastical sites, where free-standing stone or timber crosses marked significant spaces within the monastic enclosure. That this particular base has ended up sheltered inside a building on the island, rather than in situ in the ground, suggests it was recovered or moved at some point, though the circumstances of that relocation are not recorded.